Pulmonary embolism (PE) is a common health problem and a leading cause of death in all age groups. Most pulmonary emboli result from deep vein thrombosis (DVT) in the lower extremities or pelvis. The blood clots that form in another part of the body can migrate through the veins back to the heart and into the lungs, leading to a pulmonary infarct by depriving the blood and oxygen supply to a portion of the lung. An important risk factor for the development of DVT is venostasis; common scenarios include bedridden trauma patients and passengers on long airplane flights. Other causes of DVT are hypercoagulability and vessel wall inflammation. Corriere M, et al. Vena cava filters: an update. Future Cardiol. 2(6): 695-707 (2006).
Untreated PE is associated with a high mortality rate, generally held to be about 30%, with 11% of patients dying within the first hour. Patients with recurrent PE are at much higher risk. However, when the condition is promptly treated, the survival rate increases significantly. Pulmonary embolism [on-line]. Retrieved on Jul. 11, 2008 from http://www.mayoclinic.com/health/pulmonary-embolism/DS00429/DSECTION=complications. Anticoagulant therapy, such as heparin and warfarin, is the first line of treatment for PE. For patients in whom anticoagulation is contraindicated or inadequate, such as trauma and cancer patients, vena cava filters, including inferior vena cava (IVC) filters, provide alternative protection from PE. Corriere M, et al. Vena cava filters: an update. Future Cardiol. 2(6): 695-707 (2006). Vena cava filters are typically metal devices deployed under fluoroscopic guidance into the vena cava to prevent blood clots from migrating to the lungs. An IVC filter is usually placed below the level of the renal veins with the tip above the outflow of the renal veins. When the blood clot is captured in the top of the filter, it is then washed and lysed by the influx of the blood flow.
While some vena cava filters are permanently placed in the patient, there are potential complications associated with long-term filter implantation, including thrombotic occlusion of the vena cava, filter migration, filter fragmentation and filter embolization. Mohan C, et al. Comparative efficacy and complications of vena caval filters. J. Vasc. Surg. 21:235-246 (1995). See also U.S. Pat. No. 7,261,731. Nonpermanent filters, including temporary and retrievable filters, are recommended for patients having a limited period of risk for PE or the contraindication to anticoagulation. These types of filters are also recommended in adolescent and young-adult patients with normal life expectancy. Linsenmaier U et al. Indications, management, and complications of temporary inferior vena cava filters. Cardiovasc. Intervent. Radiol. 21(6): 464-469 (1998). Some temporary vena cava filters are attached to a wire or catheter, which is either exteriorized or secured subcutaneously for filter removal. The peripheral tether causes a certain degree of patient immobility and increases the risk of infection. Murray A, et al. Radiology 225:835-844 (2002).
In U.S. Pat. No. 6,391,045, a vena cava filter is disclosed that comprises a set of helical filter-wires joined at a central region and terminating in free ends constructed to engage the vessel wall. A major mid-portion of the length of the free-ended wires are generally helical forming shape. Anchoring is accomplished by a separate assembly formed of struts and anchoring devices. A trapezoid supporting strut assembly and other means for providing linear engagement with the wall of the vena cava are also disclosed. U.S. Pat. No. 6,059,825 discloses a retrievable vena cava filter formed of a single high-memory wire. The wire has a coiled cylindrical portion and a coiled conical portion. The coils of the cylindrical portion have a sufficiently large diameter contact the walls of the inferior vena cava with sufficient force to hold the coils in place against the inferior vena cava. The conical portion of the wire has a segment that aids in the removing of the filter from the vena. The vena cava filter of U.S. Pat. No. 5,954,741 features an inflatable balloon at or near the distal end of an elongate flexible multiple-lumen core or stem. The balloon is deflated prior to insertion; it is inflated to become a filter when properly positioned in the vein, and finally it is deflated for removal purposes.
In the U.S.A., there are currently six FDA-approved permanent vena cava filters with different shapes, configurations, sizes and materials. They include the stainless steel Greenfield filter (Boston Scientific, Natick, Mass.), the Bird's Nest filter (Cook, Bloomington, Ind.), the Simon Nitinol Filter (Bard, Tempe, Ariz.), the TrapEase filter (Cordis, Miami Lakes, Fla.), the Vena-Tech filter (B. Braun Medical, Evanston, Ill.) and the G2 filter (Bard, Tempe, Ariz.). There are only two FDA-approved retrievable vena cava filters: the Gunter-Tulip filter (Cook, Bloomington, Ind.) and the OptEase filter (Cordis, Miami Lakes, Fla.). Corriere M, et al. Vena cava filters: an update. Future Cardiol. 2(6): 695-707 (2006).
Retrievable vena cava filters are designed with specific features, so depending on the individual situation, they may either be left in the vessel permanently or be retrieved. While the versatility of retrievable filters makes them favorable options, in clinical practice, a large number of the retrievable filters are prone to migration and tilt. Filters have been reported to migrate to the heart, pulmonary vasculature, and distally, along with subsequent vascular perforation due to filter strut extrusion. Cunliffe C, et al. A fatal complication of a vena cava filter associated with pulmonary thromboembolism. Am. J. Forensic. Med. Pathol. 29: 173-176 (2008). Filter tilt seriously reduces the filtering efficiency. The tilt of greater than 14 degrees from the longitudinal axis is considered to be associated with the increased incidence of recurrent PE. Joels C, et al. Complications of inferior vena cava filters. Am Surg. 69:654-659 (2003). The migration or tilt further makes it difficult or impossible to retrieve the filter.
It is, therefore, desired to develop a retrievable vena cava filter that has high filtering capacity with no impedance to flow, is securely fixed on the vena cava wall (non-migrating and non-tilting), and can be easily retrieved. It is also advantageous to develop a retrievable filter than can be deployed at the patient's bedside without the need of fluoroscopy.